Comparing the rifle calibers, I was mainly focusing on your lumping the trajectory of the 6.8spc with the 30-30 and the 7.62X39. This is not a correct comparison. Here is one handy table to check:
http://www.chuckhawks.com/rifle_trajectory_table.htm
Here are some common calibers, and their “max point blank range” which is the greatest distance the bullet will travel between 3” above and below the bore. This is a shorthand method for quantifying the flatness of a trajectory, from rainbow to laser.
.223 55gr at 3240fps MPBR 239 yards
.243 110gr at 3100fps MPBR 283 yards
6.8 115gr at 2800fps MPBR 267 yards
30-30 150gr at 2390fps MPBR 225 yards
7.62X39 123gr at 2365 MPBR 225 yards
7.62NATO 150gr at 2800 MPBR 275 yards
7.62NATO 180gr at 2610 MPBR 259 yards
30-06 180gr at 2700 MPBR 269 yards
So you can see, in terms of trajectory, the 6.8spc is up with the 7.62NATO and 30-06, not the 30-30 or 7.62Russian. On the battlefield, this flatter trajectory translates into a much longer effective range. A different table showing drops at longer ranges would show the 30-30 and 7.62 having so much greater drop that they would be categorized as having rainbow trajectories. Not so with the 6.8, 7.62NATO or 30-06.
Neat info-
The 556 62 grn does not do 3240-more like 2750-2800 from an M4
The 6.8 fails to meet that 2800 fs as well-more like 2450-2500 from a 14 inch M4
Got to compare apples to apples. Remember what I stated in my post-’No free lunches for the Dogs of War”.
It would be nice if the 6.8 actually did perform that well from a 14 inch M4-the truth is not in your quoted table, likely it was derived using a 24 inch barrel or maybe even longer. 10 inches of added length would add approx. 25-40 fs per inch, or 250-400 more velocity than a 14 inch tube. I would not expect any more than my estimated 2450-2500 fs.
Also, given the design of the M4, the point blank figures should be using 14-15 inches above and below the point of aim-the military expects troops to aim center mass and on a standing enemy, that total target height is around 28-30 inces (hips to collar bones)
The M16A2 point plank range is 360 m with M855 ammo (rear sight set at 300+1 click, zeroed at 42 m, then RS backed to 300) Bullet path is approx 15 inches high at 250m, crosses line of sight at 300m and falls below 15” low at approx 360m. M4 5.56 is zeroed in similar fashion, with max PB at around 300m IIRC.
See FM 23-5 M16/A1/A2, M4, M4A1 Rifle Marksmanship for data tables etc. I do not know of any comprehensive M4/6.8 data, just ballistic software figures.
My estimation for the 6.8/M4 is that is would not significantly add PB range, compared to the M855/M4 (again, same platform, different caliber).
Would be interseting to see actual test data.